This invention relates to a flat gasket, particularly to a cylinder head gasket for internal combustion engines. The gasket has at least one aperture; the gasket edge defining the aperture is protected by a sheet metal border flange which constitutes a frame enclosing the aperture. In the zone of the sheet metal border flange--hereafter frame--on both sides of the gasket, planar sheet metal overlays are provided.
Gaskets made of sheet metal or plates of soft material, such as cylinder head gaskets used in internal combustion engines, have apertures which are in precise alignment with an engine cylinder (combustion chamber) formed by complemental cavities in the cylinder head and in the cylinder block tightened to one another with the interposition of the gasket. Such apertures are metal-framed to protect the gasket zones, for example, against the effect of the hot combustion gases in the cylinders or they are provided for the purpose of providing an increased sealing pressure at the gasket zone around the opening, since such a zone is exposed to particularly large stresses. It is further known, as disclosed, for example, in German Pat. No. 1,074,341, to cover certain other gasket zones with sheet metal overlays, for equalizing the sealing pressure distribution. Further, such overlays are necessary for protecting the gasket in the immediate vicinity of the gasket openings associated with the combustion chambers in diesel engines of current design, since such engines have pre-combustion chambers which are provided in the cylinder head and which, during the operation of the engine, are heated to a red glow at their bottom supported on the cylinder head gasket. Further, these locations are exposed to substantial mechanical stresses, since the half-combusted gases in the pre-combustion chambers flow, through single-hole nozzles, with a very high velocity into the respective cylinders.
Heretofore the protective metal overlays have been formed in the desired zones of the gasket by flap-like extensions of the upper rim portion (upper leg) of the frame, so that in a single operation a framing of the aperture and a protective overlay could be provided. Such a procedure however, does not provide an overlay on the other, bottom side of the gasket which is oriented toward the cylinder block. Thus, as a result, in this zone the sealing pressure is reduced. An apparently obvious remedy, namely an extension of the lower frame rim over zones of the gasket underface is not feasible, because during the assembly of the gasket structure, upon bending the second flap, substantial stresses and expansions would appear because of the circularly extending frame. Such stresses could lead to a breakage of the frame. Therefore, in practice, that face of the cylinder head gasket which is oriented towards the engine block has been provided with a separately mounted and secured metal overlay. Thus, in view of the above, either such overlay had to be dispensed with, in which case an optimal sealing pressure could not be obtained, or a separate overlay had to be provided which, in turn, involves a relatively complex and expensive procedure.